Cover photos (clockwise from top): Mill Bridge, Alton-Old Town; Presumpscot Falls Bridge, Falmouth; Lows Covered Bridge, Guilford-Sangerville; and Middle Branch Bridge, T05 R09 (Ebeemee)
Prepared by Guertin Elkerton & Associates
AUGUST 2003 Maine Department of Transportation
Chapter 1
GENERAL
Deer Isle-Sedgwick Bridge, Deer Isle-Sedgwick
Bailey Island Bridge, Harpswell CHAPTER 1 - GENERAL
1 GENERAL...... 1-1 1.1 Introduction...... 1-1 1.2 General Team Approach Guidelines ...... 1-3 1.3 Final Design Issues ...... 1-3 1.3.1 Plans, Specification and Estimate (PS&E) ...... 1-3 1.3.1.1 Plans ...... 1-3 1.3.1.2 Structural Design Computations...... 1-3 1.3.1.3 Geotechnical Design Computations ...... 1-4 1.3.1.4 Bridge Ratings...... 1-4 1.3.1.5 Special Provisions ...... 1-4 1.3.1.6 Engineer’s Estimate ...... 1-4 1.3.1.7 Bridge Information Form...... 1-7 1.3.1.8 Budgetary Information ...... 1-7 1.3.2 Maintenance of Traffic...... 1-7 1.3.3 Survey Layout ...... 1-7 1.4 Design Check Guidelines ...... 1-7 1.5 Small Bridge Initiative ...... 1-11 1.5.1 Field Survey Considerations...... 1-11 1.5.2 Right-of-Way Considerations...... 1-11 1.5.3 Geotechnical Considerations...... 1-11 1.5.4 Hydrologic Considerations...... 1-12 1.5.5 Minimization of Approach Work...... 1-12 1.5.6 Reduction of Structural Design Effort ...... 1-12 1.5.7 Contracting Strategies...... 1-13 1.6 Non-Vehicular Bridges...... 1-14 1.7 Aesthetics...... 1-15 1.7.1 General...... 1-15 1.7.2 Design Considerations ...... 1-15 1.7.2.1 Superstructure...... 1-16 1.7.2.2 Substructure...... 1-16 1.7.2.3 Color...... 1-21 References ...... 1-24
Table 1-1 Type/Material Selection Guide for Small Bridge Projects...... 1-13
Figure 1-1 Consistent Use of Flares ...... 1-17 Figure 1-2 Methods to Thin Appearance of Fascia ...... 1-18 Figure 1-3 Effect of Overhang Length on Beam Shadow...... 1-19 Figure 1-4 Hammerhead Pier Proportions ...... 1-19 Figure 1-5 Variations of Cantilever Length and Batter ...... 1-20 Figure 1-6 Ratio of Pier Width to Fascia Depth...... 1-22 Figure 1-7 Effect of Column Shape on Shadows and Thin Appearance...... 1-23
August 2003 i CHAPTER 1 - GENERAL
1 GENERAL
1.1 Introduction
This document is intended to provide guidance to those performing design for the Bridge Program of the Maine Department of Transportation (MaineDOT). It should provide clarity to the design thought process, and serves as a supplement to the applicable AASHTO standards. It should be used in conjunction with good engineering judgment.
This document is a companion volume to the MaineDOT “Project Management Guide” and the “Plan Development and Estimating Guide.”
The Mission and Goals of the Bridge Program are on the following page.
August 2003 1-1
Mission
The Bridge Program delivers safe, cost effective, quality bridge projects to our customers on schedule.
Goals and Objectives
1. Reduce backlog of deficient bridges
o Comprehensive planning effort to prioritize bridge needs o Resources to deliver the Bridge Program
2. Ensure project timeliness
o Complete construction on schedule o Meet project schedule needs
3. Assure project quality and cost effectiveness
o Provide quality projects that meet the purpose and need at optimum cost o Improve staff effectiveness through continuous employee development
4. Foster public satisfaction
o Share information, seek public input, and build public trust
1-2 CHAPTER 1 - GENERAL
1.2 General Team Approach Guidelines
The Bridge Program is regionally organized into Self-Directed Work Teams (SDWTs), each led by a Project Manager. In addition to the Project Managers, each team is composed of Structural Designers, Design Technicians, a Geotechnical Designer, Construction Residents, Construction Inspectors, a Utility Coordinator, a Mapper, an Appraiser, and a Team Coordinator. The environmental coordination function is managed by the Environmental Coordinator from MaineDOT’s Environmental Office, while survey functions are managed by the Survey Coordinator within the Program.
Each team member has a specific role that is integral to the success of the project as it moves through the project development process. The Structural Designer and the Geotechnical Designer provide the design expertise, and use the resources of the team to provide input into the decision-making that is part of every design.
1.3 Final Design Issues
1.3.1 Plans, Specification and Estimate (PS&E)
This documentation includes a package of information that is used to prepare the bid documents for advertising a project. The package is prepared by the project team and further assembled by the Contracts Technician within the Program. It includes the following items, with the responsibility of the Designers noted:
1.3.1.1 Plans
The plans consist of complete contract drawings that adequately display the design with enough detail to construct the project. The plans are the responsibility of the Design Technician, but must be reviewed by the Designers for conformance to the design. During the development of the plans, communication is essential to avoid rework. Standard notes are found in Appendix D. Plan layouts and detailing practices can be found in the “Plan Development and Estimating Guide.”
1.3.1.2 Structural Design Computations
Detailed design computations from the selected alternate are bound, dated, and submitted by the Structural Designer as part of the PS&E package. Design computations should include all references and assumptions used during design. After submission, they are retained in the Computations file cabinet of the Bridge Program.
August 2003 1-3 CHAPTER 1 - GENERAL
1.3.1.3 Geotechnical Design Computations
Geotechnical design computations are included as an appendix of the Geotechnical Design Report. Design computations include all references and assumptions used during design. After completion of the project, the geotechnical file is retained in the Materials, Testing, and Exploration archives in Bangor.
1.3.1.4 Bridge Ratings
Each bridge must be rated by the Structural Designer with a live load rating. Currently, bridges are being rated by the LFD method. Refer to the Manual for Condition Evaluation of Bridges 1994, with interims thru 2001, for guidance in the live load rating calculation.
1.3.1.5 Special Provisions
In most cases, Supplemental Specifications, commonly used Special Provisions, and/or project specific Special Provisions will be necessary to complement the Standard Specifications. Current Supplemental Specifications and commonly used Special Provisions are available for review. The Designers review and format these specifications for necessary inclusion in the contract documents. If project specific specifications are warranted, the Designers write and format them for the PS&E Package. The Project Manager may be involved in writing some project specific specifications that are not design related.
1.3.1.6 Engineer’s Estimate
This confidential document consists of a detailed estimate of quantities and costs necessary to construct the project. Typically, the Design Technician, with input from the Designers and Project Manager, develops the pay item list and computes the estimated quantities. The Design Technician then inputs the quantities into ESTIMATOR, which will provide automatic weighted average costs for each of the pay items. The Designers are responsible for reviewing those costs and adjusting them where needed, using engineering judgment. For a complete guide to developing an estimate or check, refer to the Bridge Program’s “Plan Development and Estimating Guide.”
August 2003 1-4
Bridge Information Form Project Bridge Parameters PIN Number of Spans Location Span Configurations ft Bridge Number Bridge Length (CL Brg Abut to CL Brg Abut) ft Project Manager Skew º Lead Designer Bridge Width (Fascia to Fascia) ft Lead Technician Roadway Width (Curb to Curb) ft Resident Buried Structure Barrel Length ft Beam Spacing ft Design Code Slab Thickness in LFD Approach Length (excluding bridge) ft LRFD Scope Work Attribute BIKEWAY Consultant LARGE BRIDGE CONSTRUCTION-NEW Consultant MEDIUM BRIDGE CULVERT REHABILITATION Consultant SMALL BRIDGE CULVERT REPLACEMENT Over Water Replacement X-LARGE BRIDGE DECK REHABILITATION Over Water Replacement LARGE BRIDGE DECK REPLACEMENT Over Water Replacement MEDIUM BRIDGE IMPROVEMENT Over Water Replacement SMALL BRIDGE PAINTING Over Water Replacement X-SMALL BRIDGE RAIL & CURB IMPROVEMENT Overpass Replacement LARGE BRIDGE REHABILITATION Overpass Replacement MEDIUM BRIDGE REMOVAL Rehab X-LARGE BRIDGE REPLACEMENT Rehab LARGE BRIDGE SUBSTRUCTURE REHABILITATION Rehab MEDIUM BRIDGE SUPERSTRUCTURE REPLACEMENT Rehab SMALL BRIDGE WEARING SURFACE REPLACEMENT Paint SIMPLE BRIDGE WIDENING Paint COMPLEX TEMPORARY BRIDGE Other (explain)______Other (explain)______
Bridge Information Form Estimated Quantities Pier Type Volume of Abutment Concrete yd3 Mass Volume of Pier Concrete yd3 Pile Bent Volume of Rigid Frame Concrete yd3 Hammerhead Volume of Structural Slab Concrete yd3 Shaft Total Length of Concrete Beams/Girders ft Other (explain)______Weight of Structural Steel lb Weight of Bituminous on Bridge lb Pier Foundation Type Weight of Substructure Rebar lb H-Pile Weight of Superstructure Rebar lb Pipe Pile Spread Footing Abutment Type Spread Footing on Bedrock Stub Cantilever Drilled Shaft Medium Cantilever (5'
CHAPTER 1 - GENERAL
1.3.1.7 Bridge Information Form
The form preceding this section is completed by the Structural Designer as part of the PS&E package. It is available electronically as an Excel spreadsheet, and is used to establish a reliable database for tracking project features and preliminary estimate costs, and for adjusting costs in Engineer’s Estimates.
1.3.1.8 Budgetary Information
In addition to the Engineer’s Estimate, there are several documents that must be completed to ensure that the updated costs of the project are distributed throughout the MaineDOT. The Project Manager completes other budgetary forms, including the Project Cost Summary Form, Construction Authorization Form, and the portion of the PS&E form that pertains to costs. These forms can be found in the Project Management Guide.
1.3.2 Maintenance of Traffic
A Traffic Control Plan must be developed for every project. Responsibility for this plan is with either the Contractor, or MaineDOT, as determined at the PS&E stage. The complexity of the project may steer the Structural Designer toward keeping this responsibility within MaineDOT, to assure compliance with the conceptual design. Any traffic control plan must comply with the latest edition of the Manual of Uniform Traffic Control Devices (MUTCD).
1.3.3 Survey Layout
A DAB (describe alignment bearing) Report or similar geometric output file should be submitted by the Designer as part of the PS&E package. This file is used in conjunction with the horizontal alignment files to generate all necessary field layout information. For a more comprehensive description of required information, please refer to the “Bridge Plan Development and Estimating Guide.”
Currently, MaineDOT provides Contractors with horizontal and vertical project control and quality assurance only. The Contractor is responsible for all remaining construction survey activities.
1.4 Design Check Guidelines
As a general rule, the design check of a structure or foundation will be assigned to a Structural Designer or a Geotechnical Designer, respectively (Design Checker). The check and/or review of the construction plans and the Engineer’s
August 2003 1-7 CHAPTER 1 - GENERAL
Estimate will be assigned to a Design Technician (Detail Checker or Reviewer). Design checks should be completed before any structural detailing is done whenever possible. Additional Structural/Geotechnical Designers and Design Technicians may be assigned to assist in the checking and review process for more complex projects or to facilitate project schedules. Occasionally, at the Team’s discretion, the Design Checker and Detail Checker or Reviewer may be the same person.
There are six general areas where checking and/or review of a project should occur and these are:
o Preliminary Design Reports
o Geotechnical Design Reports (including Series 100 Reports)
o Hydrology/Hydraulics/Scour
o Final Structural and Approach Design of In-House Projects
o Final Structural and Approach Design of Consultant Projects
o Shop Drawings
The Structural or Geotechnical Designer (Designer) is responsible for a cost effective and efficient design in accordance with this “Bridge Design Guide” and the Preliminary Design Report (PDR). The Design Checker is responsible for assuring that this goal was met. The Designer is then responsible for communicating the design parameters and configuration to the Design Technician. The function of the Design Checker is not to re-design a project, but to perform the expected level of check or review as follows:
o Independent Design Check: Perform an independent structural or geotechnical analysis of designed components to assure that the design criteria are met. This level of design check is appropriate for structural and geotechnical components of new and rehabilitated structures, and horizontal and vertical geometry of approaches.
o Design Review: Use engineering judgment to evaluate the design of structural and geotechnical components without performing a structural analysis. This level of design review is appropriate for geotechnical reports (including Series 100 Reports), hydrology and hydraulics, consultant PDRs, consultant final designs, and structural notes.
PDRs are subject to the team process in which Coachpoint meetings and consultations with Team Members, municipalities, state and federal agencies, peers, and Functional Managers provide feedback and direction for the project. A completed PDR is reviewed and approved by the Engineer of Design for its
August 2003 1-8 CHAPTER 1 - GENERAL design recommendations, and by the Assistant Program Manager for its budget, prior to the general distribution of the PDR for comments. The hydrology, hydraulics, and scour for a project should undergo a design review.
When a design is being performed by a new or inexperienced Structural Designer, the Design Checker should be an experienced Structural Designer. Inexperienced Structural Designers may be assigned as the Design Checker for designs done by experienced Structural Designers. All Geotechnical Reports should be checked and reviewed by experienced Geotechnical Designers only.
The Design Technician is responsible for developing good quality construction plans that will accurately communicate the Designer’s vision to the Construction team members and to the Contractor. The Detail Checker or Reviewer is responsible for assuring that this goal was met. The function of the Detail Checker or Reviewer is not to re-detail a project, but to perform the expected level of check or review as follows:
o Significant Detail Check: Verify significant details of major components and review completeness of plans (are there adequate sections, plan views, elevations, etc.). This level of detail check is appropriate for such items as approach plans, structural details of new and rehabilitated structures, foundation details, boring sheets, and estimated quantities.
o Detail Review: Use engineering judgment to evaluate the details without performing verification calculations, and review completeness of plans. This level of detail review is appropriate for such items as wearing surface projects, structural plate projects, reinforcement schedules, pay item lists, general notes, and consultant final plans.
The quality of a project begins with the Structural Designer, Geotechnical Designer, and the Design Technician. It is their responsibility to produce the preferred level of accuracy and completeness. They should not rely on the Project Checkers and/or Reviewers to fill in the missing pieces.
The Checkers and/or Reviewers should be aware of any commitments to town officials or other agencies to assure that they have been incorporated into the design of the project. The Design Checker should note all the changes that he/she feels are necessary for the Designer’s consideration. The Design Checker may also point out where the Designer could have used better judgment in design concepts, structural features, or structural economy. At times, a poor practice employed by the Designer may be allowed to stand in order to expedite the project. However, such poor practices, even if they are not of great consequence, should be pointed out to the Designer for his/her own benefit in order to prevent future repetition of that poor practice.
August 2003 1-9 CHAPTER 1 - GENERAL
The Detail Checker should note all the changes that he/she feels are necessary for the Design Technician’s consideration, if such changes may result in a significant cost reduction impact or if there is a risk of construction error. The Detail Checker should recommend a plan layout change only if there is a risk of construction error. If construction plans are poorly organized and difficult to decipher, then the Detail Checker should bring this to the Design Technician’s attention for future reference. After the check/review process is completed, the Designer should inform the Detail Checkers of any additional changes made to the construction plans as a result of comments received from other programs, agencies, or the Engineer of Design.
When plans have been developed by new or inexperienced personnel, the Detail Checker or Reviewer should be an experienced Design Technician, Structural Designer, or Resident. The level and extent of detail check should be increased accordingly, due to the increased potential for omissions and errors. Inexperienced Design Technicians may be assigned as Detail Checker or Reviewer on plans developed by experienced Design Technicians.
If a dispute occurs, the disputants (whether they are the Design Checker and the Designer, or the Detail Checker and the Design Technician) should attempt to resolve the dispute themselves, consulting with their peers as the need arises. If an agreement cannot be reached even after consultation with their peers, then the case should be presented to an arbiter appointed by the Engineer of Design.
This same procedure applies if there is a disagreement between the Designer and the Design Technician. Past practice has been that the Designer has final say on the project’s plans. Designers and Design Technicians should respect each other’s professional skills and knowledge in their areas of expertise.
A 2% to 5%+ margin of error is acceptable for design overstress for either the superstructure or substructure design. A 10% to 15%+ margin of error is acceptable for design understress before a design reduction is recommended. These percentages depend greatly on the cost impacts and on the uncertainty of the design assumptions. For example, if the Structural Designer proposes to use #6 bars at 6” and the Design Checker finds that this is 20% overdesigned and that #5’s and #6’s alternating at 6” will probably work, the overdesign may be preferred for its simplicity in rebar detailing, ordering, and placement.
Margins of error for dimensions of significant details vary depending upon the structure component and type. For structural steel, the margin of error may be from 1/8” to 1/2”. For camber dimensions, the margin may be from 1/8” to1/4”. Blocking dimensions should be within 0.02 feet. A 1/4” to 1/2” margin of error is acceptable for cast-in-place concrete and a 1/8” to 1/4” margin of error is acceptable for precast, prestressed concrete. For cast-in-place concrete substructures, the nearest 1/2” is acceptable.
August 2003 1-10 CHAPTER 1 - GENERAL
1.5 Small Bridge Initiative
A reduced project delivery process should be considered for any bridge project with a structure of 50 foot span or less.
These small bridge projects may not need a full hydrologic analysis, complete topographical field survey, right-of-way takings, utility adjustments, public meetings, subsurface investigations, or other activities typically used for larger projects. If a reduced process is considered, the project team should conduct a site review to determine the degree of effort and the scope of work. Discussions should also take place with abutting property owners and municipal officials.
1.5.1 Field Survey Considerations
Project characteristics that favor limited or no survey include:
o Rural setting with few manmade features near the bridge
o No permanent right-of-way acquisitions
o In-kind structure replacement with very limited approach work
o Acceptable existing roadway geometry
o No sensitive environmental resources needing to be mapped
o Lack of critical cross sectional issues
1.5.2 Right-of-Way Considerations
If practical, project limits and scope can be adjusted to require only work permits or construction easements.
1.5.3 Geotechnical Considerations
The Geotechnical Designer should assess the need for a geotechnical subsurface investigation. The Geotechnical Designer should collect previous subsurface data, field observations, performance data of the existing substructure, and typical soil characteristic tables to make a site-specific decision. In the event that enough information regarding the subsurface conditions exists, the Geotechnical Designer may choose to eliminate the subsurface investigation.
Even when the subsurface investigation is eliminated, design considerations (i.e., bearing capacity, settlement, frost protection, etc.) should be assessed by the Geotechnical Designer and made a part of the permanent record. When the subsurface investigation is eliminated from a project, it should be
August 2003 1-11 CHAPTER 1 - GENERAL
understood that this will result in the need for a more conservative design and the use of higher factors of safety. The use of higher factors of safety may, in the end, be more economical than performing an in-depth subsurface investigation.
1.5.4 Hydrologic Considerations
Project characteristics that favor limited or no formal analysis of hydrology and hydraulics are found in Section 2.3.3 Level of Analysis.
1.5.5 Minimization of Approach Work
Limits of approach work, approach roadway width, guardrail upgrades, and surface treatments should be consistent with the adjacent roadway. Relaxation of design standards should be considered to achieve this consistency. The project length should be kept to an absolute minimum.
In considering relaxing these standards, the Designer should check with the Regional Transportation Advisory Committee (RTAC) representative in the Bureau of Planning to be certain that the corridor is not likely to be upgraded in the near future.
1.5.6 Reduction of Structural Design Effort
Structure type should be determined from Table 1-1 whenever feasible, instead of performing cost comparisons of various alternates in the Preliminary Design Report. Structures that do not meet the criteria would need to be custom designed.
A substructure should be designed to minimize stream impacts whenever possible, in view of typical short in-stream work windows. Consider using longer spans by placing the abutment behind an aging abutment that can adequately support the embankment, or choosing a replacement structure that does not require in-stream work. Minimize necessary work in the stream by founding the abutment above frost, if minor movement can be tolerable, or by choosing low impact structure types, such as pile bents or drilled shafts.
August 2003 1-12 CHAPTER 1 - GENERAL
Table 1-1 Type/Material Selection Guide for Small Bridge Projects Structure Type
Bedrock at Span Range Structure Type Determination Site Go to Materials Bedrock Plate Arch or Frame Determination 10 to 21 ft No Bedrock or Go to Materials Pipe, Pipe Arch, or Box Easily Removed Determination Go to Materials Bedrock Frame Determination 22 to 26 ft No Bedrock or Go to Materials Box Easily Removed Determination Concrete Arch, Concrete Frame, or 26 to 50 ft NA Concrete Voided Slab
Structure Material
Water or Soil Reactivity Maintenance of Salt or If Existing Material Soil or Traffic During Brackish Pipe is Steel, Determination Water pH Construction Water? Age? Yes 5 to 9 NA Close Road Aluminum Yes 5 to 9 NA Staged Concrete Yes <5 or >9 NA NA Concrete
No 6 to 8 < 40 years Close Road Aluminum No 6 to 8 < 40 years Staged Concrete Galvanized No 6 to 8 > 40 years Close Road Steel No 6 to 8 > 40 years Staged Concrete No 5 to 9 NA Close Road Aluminum No 5 to 9 NA Staged Concrete No <5 or >9 NA NA Concrete
1.5.7 Contracting Strategies
The following strategies should be considered to reduce construction costs:
o Grouping small projects for advertising can reduce costs. The projects should be located geographically near each other for efficiency of both MaineDOT personnel and the Contractor, and should be of similar scope. Projects from another Program sharing the same highway corridor or in the same general vicinity should also be constructed under one contract when feasible.
August 2003 1-13 CHAPTER 1 - GENERAL
o Simplify project details to allow for faster construction, especially for projects with short project schedules. Examples include the use of integral abutments, elimination of bridge skews, use of prefabricated superstructure elements, using uniform details, etc.
o Time the bidding to allow enough time for the Contractor to plan their work. The advertisement of grouped projects should be far enough in advance of the construction season to allow as many Contractors to bid as possible.
o Consider a reduced plan or no plan project. The project should have a well-defined scope, such as replacing an existing pipe with another pipe or pipe arch. There would be no survey obtained, and the plans would include: a typical pipe or pipe arch sheet, a typical roadway cross-section, and typical guardrail end treatments. These plans would be on standard letter size sheets that are inserted into the contract proposal book. For these projects, sufficient right-of-way must be available or easily attainable to construct the project, and minimal environmental impacts must be anticipated.
1.6 Non-Vehicular Bridges
A multi-use bridge may be constructed for a combination of pedestrians, bicyclists, snowmobiles, or other users. For loading criteria, refer to Section 3.8 Non-Vehicular Bridges. Prefabricated pedestrian bridges must be designed by a registered Professional Engineer.
The owner and maintainer of the bridge should consider the following issues when developing the design:
o Width - For guidance on how wide a trail bridge should be, refer to AASHTO “Guide for the Development of Bicycle Facilities.” A width less than 10 feet will prevent most vehicles from getting on to the bridge except for snowmobiles, ATV’s, golf carts, and motorcycles. If the bridge will be plowed, additional width may be necessary.
o Vertical clearance - Vertical clearance is an issue with timber covered bridges or box type steel trusses. The minimum vertical clearance is 8 feet. Low vertical clearance will prevent heavier vehicles from using a bridge. A high vertical clearance of 14’-6” or more may be needed to accommodate snow grooming equipment, occasional maintenance equipment, or emergency vehicles.
o Emergency Vehicles – If emergency vehicles (ambulances, fire trucks, etc.) are expected, they should be accommodated. The bridge may be the only access to a remote area.
August 2003 1-14 CHAPTER 1 - GENERAL
o Inspection/Maintenance - How will the bridge be inspected and repaired? Refer to Section 2.9.6 Maintainability.
o Bollards – Bollards may be used to control or limit access. Bollards are usually timber or steel posts spaced at about 5 foot spacing that prevent large vehicles from going onto a bridge. The spacing of the bollards can be reduced to 3 feet clear to prevent virtually all motorized vehicles from using the bridge. Removable bollards should be considered if emergency vehicles will occasionally use the bridge.
o Rail - Bridges that may be used by snowmobiles should use at least a 54” bicycle height bridge rail. The use of a rub rail is highly recommended to prevent bicycle handlebars from catching on the bridge rail. The center of the rub rail should be 3’-6” above the riding surface.
The Structural Designer should also consider the use of security fencing, lighting, and attached utilities on the bridge. The load capacity of the bridge should be clearly posted on or near the bridge in accordance with MUTCD.
1.7 Aesthetics
1.7.1 General
Aesthetics involves more than just surface features such as color and texture. It includes the visual and perceptual effect made by the bridge as a total structure, as well as the effect made by its individual parts. Bridges affect their surroundings by virtue of their size, shape, line, color, and texture. All structures should be designed with consideration of site-specific features to create designs that provide function as well as a pleasing appearance. The key is to create a distinguished structure without spending excessive resources.
Bridges are usually viewed from one of two places, either from the roadway as a user, or from the side. For those bridges rarely seen from the side, aesthetic considerations are limited to the appearance of the rail, sidewalk, curb, and wearing surface. For other bridges, the view of the bridge from the side should be considered in the design. The nature of the surroundings may influence the aesthetic design choices, whether the location is urban, rural, industrial, or coastal.
1.7.2 Design Considerations
Consistency in the use of flares and tapers in bridge components will result in a more harmonic structure. For example, if a column is flared to be wider at
August 2003 1-15 CHAPTER 1 - GENERAL
the top, the fascia should also be sloped. A prismatic column may look better with a vertical fascia. Refer to Figure 1-1.
1.7.2.1 Superstructure
A bridge is primarily a horizontal structure that is supported by vertical members. Fewer piers will enhance the appearance by emphasizing the horizontal line. End spans that are shorter than middle spans often have structural as well as aesthetic advantages. A constant depth superstructure will appear more graceful than one with spans of different depths. Even more graceful is a haunched girder structure, especially if the haunch transition is long, up to 40% of the span length.
The end of the slab seen on the fascia will look better if it appears thinner. This can happen by creating deeper shadows through sloping the bottom of the fascia away from the viewer, or tapering the slab thickness toward the fascia, and by using an overhang of about 2/3 the depth of the girder. Refer to Figure 1-2 and Figure 1-3.
The rail may be the most visible aspect of the bridge to the traveling public. Spending money enhancing the rail system can go far to improve the appearance of the structure. Refer to Section 4.4.6 Aesthetics.
Ornamental lighting can enhance the aesthetics of a high profile bridge. Tall light poles can be located over piers to streamline the appearance.
1.7.2.2 Substructure
Most piers are classified as short, with the length (transverse) greater than the visible height. It is more difficult to enhance the appearance of a short pier than a tall pier. The vertical nature of a tall pier can be emphasized by minimizing the batter, and by minimizing the horizontal faces of the pier by using sloped faces. When a bridge has several piers with different heights, the pier shape should be one that can accommodate varying proportions and batters to create both short and tall piers that look good. Batters can be greater on a short pier without sacrificing appearance.
Hammerhead piers should be proportioned to balance the shaft length and height, as well as the length and depth of the cantilevered cap. Some starting proportions are shown in Figure 1-4. The Structural Designer should do a scale drawing of each pier to be sure the proportions look pleasing. A short cantilever looks better when the shaft batter is negative toward the ground, while a longer cantilever is needed when the batter is positive. Refer to Figure 1-5.
August 2003 1-16 CHAPTER 1 - GENERAL
Figure 1-1 Consistent Use of Flares
August 2003 1-17 CHAPTER 1 - GENERAL
Figure 1-2 Methods to Thin Appearance of Fascia
August 2003 1-18 CHAPTER 1 - GENERAL
Figure 1-3 Effect of Overhang Length on Beam Shadow
Figure 1-4 Hammerhead Pier Proportions
August 2003 1-19 CHAPTER 1 - GENERAL
Figure 1-5 Variations of Cantilever Length and Batter
August 2003 1-20 CHAPTER 1 - GENERAL
The relative pier width (longitudinal) to the fascia depth seen from the side also affects appearance. If the pier is too narrow, the bridge will appear unsupported and weak, while a wide pier will appear bulky. The apparent fascia depth includes the parapet rail height for a closed rail system, but does not include the rail height for an open rail system. Pier width should be between 25% and 50% of the fascia depth for a concrete barrier system. It should be between 50% and 67% of the fascia depth for an open rail system. Refer to Figure 1-6.
In general, slender columns are more graceful than wider columns. Columns will look more slender if the edge facing the viewer is partially in shadow. An octagonal column may look thinner than a round column, which looks thinner than a square column. Refer to Figure 1-7.
Form liners, acid washing techniques, or stone facing can be used to create surface texture on abutments, wingwalls, and piers. If the wall is viewed only at high speeds, the patterns used must be large enough to be visible. Pay special attention to corners and tops of walls when imitating stonework with form liners. Also consider having horizontal lines on return wings such as chamfers and construction joints follow the road grade when possible.
1.7.2.3 Color
In special situations, adding color to components of the bridge can be considered to enhance the fit into the surroundings. Coloring will increase maintenance costs, and may result in a poor appearance if maintenance is neglected. Concrete can be colored, but the cost is high, quality control is difficult, and it is often hard to match colors between batches. Concrete can also be stained, which presents its own appearance and durability concerns. Steel bridge rail can be color galvanized, as discussed in Section 4.4.6 Aesthetics, Bridge Rail, and other steel structures such as historic trusses can be painted as well.
August 2003 1-21 CHAPTER 1 - GENERAL
Figure 1-6 Ratio of Pier Width to Fascia Depth
August 2003 1-22 CHAPTER 1 - GENERAL
Figure 1-7 Effect of Column Shape on Shadows and Thin Appearance
August 2003 1-23 CHAPTER 1 - GENERAL
References
AASHTO,1999, Guide for the Development of Bicycle Facilities
AASHTO,1994, Manual for Condition Evaluation of Bridges. Second Edition, with interims, Washington, DC
ATSSA/ITE/AASHTO, 2001, Manual on Uniform Traffic Control Devices, Millennium Edition, USA
CalTrans, 1993, Bridge Design Aesthetics, CalTrans Design Manual, Section 7, February
MaineDOT, Plan Development Training Manual
MaineDOT Bridge Program, Plan Development and Estimating Guide
MaineDOT Bridge Program, Project Management Manual
New York DOT, 2002, Aesthetics, New York DOT Design Manual, Section 23, April
August 2003 1-24 Chapter 2
PRELIMINARY DESIGN
Chisholm Park Bridge, Rumford
Sandy Stream Bridge, Moose River Plantation CHAPTER 2 – PRELIMINARY DESIGN
2 PRELIMINARY DESIGN...... 2-1 2.1 Preliminary Design Report...... 2-1 2.1.1 Title Page ...... 2-2 2.1.2 Table of Contents ...... 2-2 2.1.3 Background Information ...... 2-2 2.1.4 Location Map...... 2-3 2.1.5 Bridge Recommendation Form...... 2-3 2.1.6 Summary of Expected Impacts...... 2-6 2.1.7 Summary of Preliminary Design ...... 2-6 2.1.8 Existing Bridge Synopsis Form...... 2-7 2.1.9 Hydrology/Hydraulic/Scour Report ...... 2-7 2.1.10 Preliminary Plan ...... 2-8 2.1.11 Photographs ...... 2-8 2.1.12 Summary of Existing Upstream and Downstream Bridges ...... 2-9 2.1.13 Site Inspection Report ...... 2-9 2.1.14 Information Reports...... 2-9 2.1.15 Survey Plans of Existing Bridges...... 2-9 2.1.16 Hydrology/Hydraulic/Scour Data ...... 2-9 2.1.17 Miscellaneous Information...... 2-9 2.1.18 Traffic and Accident Data ...... 2-10 2.1.19 Estimates...... 2-10 2.2 Economic Comparisons...... 2-10 2.2.1 Overview ...... 2-10 2.2.2 Definition of LCCA...... 2-11 2.2.3 When to use LCCA...... 2-11 2.2.4 Deterministic Analysis ...... 2-12 2.2.5 Probabilistic Analysis...... 2-12 2.2.6 Standard Assumptions ...... 2-12 2.2.7 Cost Comparison for Number of Beams...... 2-14 2.3 Hydrology, Hydraulics, and Scour ...... 2-16 2.3.1 General...... 2-16 2.3.2 Minor Span/Strut Determination ...... 2-16 2.3.3 Level of Analysis ...... 2-17 2.3.3.1 Level 1 (Qualitative Analysis) ...... 2-17 2.3.3.2 Level 2 (Basic Analysis) ...... 2-18 2.3.3.3 Level 3 (Complex Analysis)...... 2-18 2.3.4 Data/Information Collection ...... 2-18 2.3.5 Vertical Datum...... 2-21 2.3.6 Tidal Elevation Computations...... 2-22 2.3.7 Changes in Sea Level ...... 2-26 2.3.8 Documentation ...... 2-26 2.3.9 Hydrology ...... 2-26 2.3.9.1 Introduction ...... 2-26 2.3.9.2 Discharge Rate Policy ...... 2-27 2.3.9.3 Discharge Formulae ...... 2-27 2.3.9.4 Rural Watersheds...... 2-28
August 2003 i CHAPTER 2 – PRELIMINARY DESIGN
2.3.10 Hydraulics...... 2-29 2.3.10.1 Introduction ...... 2-29 2.3.10.2 Hydraulic Analysis ...... 2-29 2.3.10.3 Discharge Velocities...... 2-33 2.3.10.4 Backwater ...... 2-33 2.3.10.5 Dams...... 2-33 2.3.10.6 Fish Passage...... 2-34 2.3.11 Scour...... 2-34 2.3.11.1 New Bridges...... 2-34 2.3.11.2 Existing Bridges...... 2-35 2.3.11.3 Riprap Slope Protection ...... 2-36 2.4 Maintenance of Traffic During Construction...... 2-37 2.4.1 General...... 2-37 2.4.2 Methods to Maintain Traffic ...... 2-38 2.4.2.1 Close the Road and Detour on Existing Roads ...... 2-38 2.4.2.2 Staged Construction...... 2-39 2.4.2.3 Temporary Bridge...... 2-39 2.4.2.4 Innovative Methods ...... 2-39 2.5 Geotechnical and Survey...... 2-40 2.5.1 Geotechnical ...... 2-40 2.5.2 Field Survey ...... 2-40 2.6 Utilities and Right-of-Way ...... 2-42 2.7 Alignments...... 2-42 2.7.1 General Highway Design Guidelines...... 2-42 2.7.2 Bridge Guidelines ...... 2-43 2.7.2.1 Horizontal Alignment ...... 2-43 2.7.2.2 Vertical Alignment ...... 2-43 2.7.3 Clearances ...... 2-43 2.7.3.1 Railroad...... 2-43 2.7.3.2 Grade Separations ...... 2-45 2.7.3.3 Underclearance for Stream Crossings ...... 2-45 2.7.3.4 Clearance Between Parallel Structures...... 2-45 2.7.3.5 Underclearances for Non-Vehicular Bridges ...... 2-46 2.8 Approaches ...... 2-46 2.8.1 Roadway Widths...... 2-46 2.8.1.1 Local Roads ...... 2-46 2.8.1.2 Collector Roads...... 2-48 2.8.1.3 Arterials ...... 2-48 2.8.2 Guardrail...... 2-48 2.8.2.1 General ...... 2-48 2.8.2.2 Guardrail Treatment on Local Roads ...... 2-48 2.8.3 Reduced Berm Offset...... 2-56 2.8.4 Pavement Design ...... 2-57 2.8.4.1 General ...... 2-57 2.8.4.2 Arterials and Collectors ...... 2-58 2.8.4.3 Local Roads ...... 2-59
August 2003 ii CHAPTER 2 – PRELIMINARY DESIGN
2.8.5 Approach Drainage...... 2-60 2.8.6 General or Local Conditions ...... 2-60 2.9 Structure Type Selection ...... 2-61 2.9.1 Span...... 2-61 2.9.2 Maintenance of Traffic...... 2-61 2.9.3 Constructability...... 2-61 2.9.4 Environmental Impact...... 2-62 2.9.5 Right-of-Way Impact...... 2-62 2.9.6 Maintainability...... 2-63 2.9.7 Historical/Archeological Issues...... 2-64 2.9.8 Cost...... 2-64 2.9.9 Aesthetics...... 2-64 References ...... 2-65
Table 2-1 Rounding Guidelines for PDR Cost Estimates...... 2-10 Table 2-2 Life Cycle Intervals ...... 2-13 Table 2-3 Design Flow versus Drainage Area and Wetland Percent ...... 2-17 Table 2-4 Clear Zone ...... 2-52 Table 2-5 Encroachment Angle...... 2-52 Table 2-6 Pavement Layer Thickness...... 2-57 Table 2-7 Number of Layers Across Roadway ...... 2-58 Table 2-8 DARWin Input ...... 2-59 Table 2-9 Pavement & Subbase Thickness ...... 2-60
Figure 2-1 Typical Railroad Cut Section ...... 2-44 Figure 2-2 NHS in Maine ...... 2-47 Figure 2-3 Point of Need Definition...... 2-50 Figure 2-4 Lateral Extent of Hazard Definition ...... 2-51 Figure 2-5 Point of Need Example...... 2-55 Figure 2-6 Lateral Extent of Hazard Example ...... 2-56 Figure 2-7 Reduced Berm Offset ...... 2-57
Procedure 2-1 Guardrail End Treatment on Local Roads ...... 2-51
Example 2-1 Cost Comparison of Number of Steel Beams ...... 2-15 Example 2-2 Datum Shift ...... 2-22 Example 2-3 Tidal Elevation at Reference Station...... 2-23 Example 2-4 Tidal Elevation at Subordinate Station...... 2-24 Example 2-5 Guardrail End Treatment on Local Roads...... 2-54
August 2003 iii CHAPTER 2 – PRELIMINARY DESIGN
2 PRELIMINARY DESIGN
2.1 Preliminary Design Report
The Preliminary Design Report (PDR) documents the justification for decisions made in the conceptual design process. Forms are available electronically that assist in completing the PDR. At the end of the preliminary design phase, all those invested in the project have reviewed the scope of work, and this scope is considered final. The PDR is then used as the starting point to proceed to final design.
For those projects with spans of 50 feet or less, consideration should be given to a reduced preliminary design effort, as discussed in Section 1.5 Small Bridge Initiative.
The PDR is organized into the following sections. The depth of study and extent of investigation of options will depend upon the complexity of the project. Samples of completed forms are found in Appendix B PDR Forms. A description of each section follows the listed sections.
1. Title Page 2. Table of Contents 3. Background Information 4. Location Map 5. Bridge Recommendation Form 6. Summary of Expected Impacts 7. Summary of Preliminary Design 8. Existing Bridge Synopsis Form 9. Hydrology/Hydraulic/Scour Report 10. Preliminary Plan 11. Photographs 12. Summary of Existing Upstream and Downstream Bridges 13. Site Inspection Report 14. Information Reports 15. Survey Plans of Existing Bridges 16. Hydrology/Hydraulic/Scour Data 17. Miscellaneous Information 18. Traffic and Accident Data 19. Estimates
August 2003 2-1 CHAPTER 2 – PRELIMINARY DESIGN
2.1.1 Title Page
The Title Page contains the following:
PRELIMINARY DESIGN REPORT BRIDGE NAME and NUMBER OVER RIVER NAME TOWN, MAINE FEDERAL PROJECT NUMBER PIN NUMBER
2.1.2 Table of Contents
This should be a properly identified index of pages.
2.1.3 Background Information
This page provides a quick reference for background information on the project. Much of this information is found either in MaineDOT’s ProjEx, the Planning Report, or Bridge Management’s SI&A sheet, all of which will be provided by the Project Team. The following sections are completed as shown below:
Program Scope: Copy verbatim the scope from the Biennial Transportation Improvement Program (BTIP).
Program Reads: Copy verbatim the contents of the project description in the BTIP.
Project Background: Provide a brief written description of the project's background, including site review by the 6-Year Plan team, any previous studies and recommendations, requests by Towns, and any other pertinent information.
Structurally Deficient: A structure is structurally deficient if the condition rating for the deck, superstructure, substructure, or the culvert and retaining wall is 4 or less. A structure may also be structurally deficient if the appraisal rating for the structural condition or waterway is 2 or less.
Functionally Obsolete: A structure is functionally obsolete if the appraisal rating for the deck geometry, under clearances, or approach roadway alignment is 3 or less. A structure may also be functionally obsolete if the appraisal rating for the structural condition or waterway is 3. Any bridge
August 2003 2-2 CHAPTER 2 – PRELIMINARY DESIGN
classified as structurally deficient is excluded from the functionally obsolete category.
2.1.4 Location Map
This should be from the Highway Atlas, U.S.G.S., or another map showing the project location. Do not use copyrighted material such as a DeLorme's Maine Atlas and Gazetteer.
2.1.5 Bridge Recommendation Form
All portions of the Recommendation Form should be completed as shown below. A complete description of each component should be included under that component. There are several variations to this form depending on the project scope. If there are parts that are not applicable to the structure type, they need not be included.
Review by - Signature of Engineer of Design is obtained here prior to proceeding with any further work.
Project - State the type of project. Examples:
“Bridge replacement with 300 ft of approaches, including transitions” “Bridge rehabilitation project with no approach work” “Bridge replacement as part of Arterial Program project” “Bridge replacement with approaches by Arterial Program”
Alignment Description - Give a description of the horizontal and vertical alignments at the structure location and the relationship to the existing alignment. Example:
"1200’ horizontal curve located approximately 30’ upstream of existing bridge and a 500’ sag (crest) vertical curve with a finish grade 3.5’ higher than existing bridge."
Approach Section - Give a description of the typical approach section at the bridge, including the type of guardrail. Example:
“Two 11' paved lanes with 3’ shoulders (30’ rail-to-rail) with standard sideslopes. 21” aggregate subbase course gravel with 3” pavement thickness. Type 3 guardrail.”
Spans - Give the span lengths along the centerline of construction on straight tangents, and along working lines or chord lines for structures on a curve. If on a curve, indicate span lengths as "along long chord" or
August 2003 2-3 CHAPTER 2 – PRELIMINARY DESIGN
other descriptive indication. This section is not required for culvert-type structures.
Skew - Give the skew angle of the substructure units, or the centerline of a culvert-type structure, relative to the longitudinal working line of the structure. The skew angle should always be given as "Ahead on Left" or "Back on Left".
Loading - Indicate the appropriate design vehicle loading.
For a typical superstructure: “HL – 93 Modified”
For a culvert-type structure: “HS 25”
Superstructure - Give the design description and governing parameters of the superstructure. For culvert-type structures, this section is simply called Structure. Examples:
For a typical superstructure: “Five rolled beams of A709/A709 M, Grade 50W steel with a composite structural concrete slab, elastomeric bearings, one compression seal expansion joint, and a 3” bituminous wearing surface with ¼” (nominal) membrane waterproofing. 36’ curb-to- curb with standard 2-bar steel rail. 2% normal crown."
For a culvert-type structure: "16’-4” span by 8’-2” rise aluminum structural plate pipe arch. Flow line of 1% with Elevation 100.00 at the centerline of construction."
Abutments - State the type of abutment and anticipated support system. Also give any specific features required. This section is not required for culvert-type structures. Example:
"Stub concrete abutments with return wings on steel H-piles, 1.75:1 (plain or heavy) riprap slopes in front" or "Deep concrete abutments with approach slabs on spread footings with sandblasted architectural facing".
Piers - State the type of piers and anticipated support system. This section is not required for culvert-type structures. Example:
"Mass concrete pier with distribution slab and concrete seal supported by steel H-beam piles."
August 2003 2-4 CHAPTER 2 – PRELIMINARY DESIGN
Opening and Clearance - For water crossings, give the total area of bridge opening and the area of bridge opening at a common elevation for both the existing and the recommended structures. The areas should be normal to the direction of flow. Also, give the minimum clearance depth at Q50 for both the existing and the recommended structures.
For overpass structures, give the minimum vertical and horizontal clearances for both the existing and the recommended structures.
For culvert-type structures, give the total opening for both the existing and the recommended structures.
Disposition of Existing Bridge - Give a brief statement of what is to be done with the existing bridge. Examples:
"To be removed to streambed, property of Contractor." "Superstructure and abutments to be removed below slope line.” “Steel beams to be retained by the Department." “Existing wearing surface, rail, and curbs to be removed.”
Available Soils Information - State what soils information was available during study or was obtained from existing plans. Also indicate if scour analysis should be made in the final design of the foundation.
Additional Design Features - Describe any design features that are not described in any other part of the Recommendation Form (e.g. something that is unusual or experimental), but which are necessary to complete the project description.
Maintenance of Traffic - State how and where traffic is to be maintained during construction of the project, whether one lane or two lanes will be required, and whether signals or flaggers will be required. Also state if maintenance of pedestrian traffic is required. If a road closure is proposed, give the detour length from abutment to abutment.
Construction Schedule - Include any restrictions and/or commitments. Example:
“One construction season with landscaping the following spring. Bridge must be reopened to traffic by Labor Day.”
Dates - For projects funded through construction, enter advertise, construction begin, and construction complete dates. For PCE-P projects funded through design, give the “Plans to R/W” date. For PCE-C projects funded through public meeting give environmental document date.
August 2003 2-5 CHAPTER 2 – PRELIMINARY DESIGN
Program Funding Level – Enter either “Construction” or PCE level
Approximate Cost - Enter the programmed, approved, and the estimated project costs under the appropriate headings.
Commentary: The estimated cost of the project is located in 4 places within the PDR: the program funding table, summary of preliminary design, preliminary plan, and the cost estimate.
Project Fiscally Approved – Signature of Assistant Program Manager is obtained here prior to proceeding with any further work.
Utilities - List the known utilities in the project limits. The utility list may be obtained from the Utility Coordinator or the utility data base.
Additional Soils Information and Additional Field Survey - Indicate whether or not the information is required.
Exception to Standards - List any exceptions to Federal or State Standards that either requires approval from FHWA (for NHS projects only), the Engineer of Design, or the Bridge Program management team via the Coachpoint process. Examples of exceptions to standards are reduced bridge widths, omitting of the leveling slab on butted precast superstructures, and reduced hydraulic clearances.
Comments - This is for comments by the Engineer of Design.
2.1.6 Summary of Expected Impacts
This form provides a summary of the expected impacts and the required permitting for the recommended project. These impacts may be right-of-way, historical, archeological, environmental, etc. The required permitting may include Coast Guard, FAA, and the various environmental permits. Filling in the required information for this form will be a project team effort.
2.1.7 Summary of Preliminary Design
This is a summary of the Preliminary Design performed to determine the project recommendations. It should describe, in an orderly fashion, the alternatives considered, with a summary of the assumptions and comparisons that are pertinent to the justification of the recommendation. It should include a discussion of bridge width, alignment, and maintenance of traffic, with the reasoning used to arrive at the recommendation. It may include a discussion of geotechnical, environmental, or utility issues, if these are pertinent to the project.
August 2003 2-6 CHAPTER 2 – PRELIMINARY DESIGN
The Summary should discuss the pros and cons of the alternatives considered and the reasons for the selection of the recommended alternative. Only the engineering that is pertinent should be discussed. The Summary should be short and to the point and should avoid superfluous and lengthy discussions.
For a water-crossing structure, reference should be made to the Hydrology/Hydraulic/Scour Report with the conclusions repeated as to the feasible structure alternatives and ultimate recommendation.
In some instances, especially on large and expensive projects, there may be several alternatives developed for public or internal review and selection. These alternatives should be summarized here, with the back-up data and calculations bound and filed elsewhere in the project file.
2.1.8 Existing Bridge Synopsis Form
This form provides a description of the physical characteristics, history, and condition of the existing structure and should be filled in as completely as possible from information in Bridge Maintenance files and project records.
2.1.9 Hydrology/Hydraulic/Scour Report
This is a summary of the hydrologic analysis that determines the design and check discharges, the hydraulic analysis that determines the structure opening and/or structure alternatives, and the scour analysis that determines the foundation requirements. Normally, this report combines the Hydrology and Hydraulics, but it can be separated into two reports if warranted. The MaineDOT Environmental Office Hydrology Unit provides a spreadsheet with the results of the U.S.G.S. full regression equation. Flows based on other methods should be computed and documented by the Designer. These flows are summarized in this section. Example:
Drainage Area 110 sq mi Design Discharge (Q50) 1240 cfs Check Discharge (Q100) 1410 cfs Scour Check Discharge (Q500) 1660 cfs Ordinary High Water (Q1.1) 380 cfs Flood of Record (Q---) 1820 cfs @ Elevation 64.3
If HEC-RAS runs will be necessary for the hydraulic study, stream slopes should be determined. If the structure is in a tidal zone, the following elevation data should also be summarized:
August 2003 2-7 CHAPTER 2 – PRELIMINARY DESIGN
Mean Lower Low Water (MLLW) -8.5 ft Mean Low Water (MLW) -8.2 ft Mean Tide Level (MTL) -0.3 ft Mean High Water (MHW) 7.5 ft Mean Higher Water (MHHW) 9.4 ft 2003 Predicted High Tide 10.7 ft
The hydraulic analysis is then discussed. Structural openings should be analyzed for flow capacity, outlet velocities, and backwater heights, using Bureau of Public Roads (BPR) charts and graphs, backwater runs, or other applicable methods. Culvert-type structures should be checked for fish passage at low flow conditions.
If no single structure alternative is obvious, the Hydrology/Hydraulic/Scour Report should describe those alternatives that are hydraulically feasible, and the final recommended alternative should be discussed in the Summary of Preliminary Design of the Bridge Recommendation Form.
A summary gives the final conclusions and hydraulic parameters. Also, for comparative purposes, the Summary should give the hydraulic parameters of the existing bridge. Example:
Existing Bridge Recommended 60 ft clear span 88 ft clear span Headwater El. @ Q50 104 ft 101 ft Headwater El. @ Q100 107 ft 102 ft Discharge Velocity @ Q50 9.1 fps 5.2 fps Discharge Velocity @ Q100 12.6 fps 6.5 fps Ordinary High Water (Q1.1) 98.1 ft 98.1 ft Discharge Velocity @ Q1.1 3.5 fps 2.0 fps Clearance @ Q50 1.3 ft 4.2 ft
2.1.10 Preliminary Plan
A half-size copy of the Preliminary Plan will be added to the PDR after its preparation and it should be included in the Table of Contents. Typical sections of existing and proposed bridges should be shown on the Preliminary Plan, as well as proposed construction and other pertinent data.
2.1.11 Photographs
A good selection of color photographs of the bridge, roadway, and stream should be taken during a field inspection visit or from photographs taken by others. Photographs may also be copied from the Bridge Maintenance files or
August 2003 2-8 CHAPTER 2 – PRELIMINARY DESIGN
obtained from local residents taken during a flood or during the construction of the existing bridge. When possible, the date the photographs were taken should be noted.
2.1.12 Summary of Existing Upstream and Downstream Bridges
Information about the upstream and downstream bridges may be useful for the hydraulic analysis. If so, they are listed here along with the size of the hydraulic opening and pertinent ice, flooding, and debris concerns.
2.1.13 Site Inspection Report
All field trips to the project site should be documented, describing all pertinent findings, conclusions, and points of interest.
2.1.14 Information Reports
Reports from Bridge Maintenance Supervisors, local residents, or Town Officials pertaining to structural condition or hydraulics should be documented. A copy of the most recent inspection report should also be included here.
2.1.15 Survey Plans of Existing Bridges
Archived survey or general plans of the existing bridge should be printed and included here. Plans of nearby bridges may also be included if they have pertinent information related to flood history, soils, or topography which could be used in the preliminary design. Pertinent structural plans may also be included for complex rehabilitation projects when deemed beneficial.
2.1.16 Hydrology/Hydraulic/Scour Data
This section provides the back-up data to the Hydrology/Hydraulic/Scour Report, such as the flow data tabulation, aerial photographs, analysis of existing bridges, FEMA data, BPR hydraulic graphs and charts, HY-8 results, HEC-RAS results, scour computations, and other relevant information. If the project has extensive computer reports from the hydraulic analysis, include the most pertinent information in the PDR. Additional hydrology/hydraulic/scour data should be compiled in a separate document, placed in the project file, and referenced in the PDR.
2.1.17 Miscellaneous Information
Any other pertinent information that is developed or obtained can be included here.
August 2003 2-9 CHAPTER 2 – PRELIMINARY DESIGN
2.1.18 Traffic and Accident Data
The traffic data information obtained from the Bureau of Planning is included here. Include accident data if pertinent to the project.
2.1.19 Estimates
Preliminary Cost Estimate forms are available electronically to assist in estimate preparations. They should be included here for all developed alternates. Supporting spreadsheets that estimate costs using detailed pay items should not be included in the PDR; however, they can be placed in the project file. As a check on the accuracy of the estimate, the square foot cost obtained should be compared to historical square foot cost data found in the Bridge Program’s Bridge Unit Cost database. All project costs should be rounded as shown in Table 2-1.
Table 2-1 Rounding Guidelines for PDR Cost Estimates Round To Item Amount Nearest: Individual construction items such as Superstructure, Cofferdams, All $1,000 Approaches, Mobilization, etc. Structure Subtotal and Approaches All $5,000 Subtotal Up to $1,000,000 $5,000 Total Construction Cost, PE, ROW, CE Over $1,000,000 $10,000
Up to $500,000 $5,000
Total Project Cost $500,000 to $1,000,000 $10,000
Over $1,000,000 $100,000
2.2 Economic Comparisons
2.2.1 Overview
During preliminary design, the Designer should consider different rehabilitation/replacement alternatives. A Life Cycle Cost Analysis (LCCA) is a tool used to select alternatives and to make economic decisions. Sound
August 2003 2-10 CHAPTER 2 – PRELIMINARY DESIGN
engineering judgment is necessary to determine input data, analyze results, and determine the relevance of the analysis.
LCCA considerations for bridges include functionality, age, condition, present costs, future costs, and present and future program funding availability. The two approaches available to evaluate LCCA are a Deterministic Analysis and Probabilistic Analysis. This section will examine both analyses.
2.2.2 Definition of LCCA
Section 303 of the National Highway System Designation Act defines LCCA as “a process for evaluating the total economic worth of a usable project segment by analyzing initial costs and discounted future cost, such as maintenance, reconstruction, rehabilitation, restoring, and resurfacing costs, over the life of the project segment”.
In short, LCCA is a method of analysis that compares the net present value of all costs related to improvements over the life of the structure. The level of detail of the analysis is determined on a project-by-project basis.
2.2.3 When to use LCCA
LCCA should be performed when comparing competing options with different life expectancies, rehabilitation costs, or maintenance costs. Common situations are listed below:
o A rehabilitation scenario for a single bridge with multiple choices such as: 1) immediate deck replacement; 2) wearing surface replacement followed in 15 years by a deck replacement; 3) deck rehabilitation and wearing surface replacement followed by a superstructure replacement in 15 years; etc. (refer to Chapter 10 Rehabilitation for a discussion of this terminology)
o Comparing a traditional bridge that has significant maintenance costs to a buried structure that has few maintenance costs
o Bridge rehabilitation compared with replacement
o Painting a bridge or waiting until the bridge is deficient and then replacing it
o Comparing steel bridge that requires painting with a concrete structure that is to be located in a harsh environment where weathering steel is not recommended
o Comparing a steel pipe to an aluminum pipe or concrete box
August 2003 2-11 CHAPTER 2 – PRELIMINARY DESIGN
2.2.4 Deterministic Analysis
A deterministic analysis is the most common method, and is adequate to evaluate LCCA in most situations. This approach compares alternatives and life cycle costs based on net present value and fixed inputs. This simplified approach will provide one solution for any given set of alternatives. To vary costs or timing, inputs need to be changed and the analysis rerun. For most projects the inputs can be easily adjusted utilizing a spreadsheet. Design examples are available in Excel from the technical resource people for economic comparisons.
2.2.5 Probabilistic Analysis
The next level of LCCA is a probabilistic analysis. This approach allows for variability and uncertainty of timing and costs. The output provides a probability of which alternate will have the lowest costs over the life of the bridge. This method of analysis is recommended for projects with significant bridge replacement or rehabilitation costs, or when the deterministic approach is insufficient.
The Bridge Program utilizes a program developed by NCHRP Project 12-43. Bridge Life Cycle Cost Analysis (BLCCA) has the ability to perform both a probabilistic and a deterministic analysis. BLCCA can be installed on the Designer’s PC as needed. A complete Guidance Manual and User’s Manual is also available for reference that can be viewed and printed through the help menus.
2.2.6 Standard Assumptions
To ensure consistency the following assumptions are recommended:
o Use a discount rate of 4%, which approximates the FHWA discount rate. This factor accounts for the annual growth rate of an investment, and does not include inflation.
o Use current and constant dollars. For example, if the cost for a repair in year 1 is $100,000, the same repair in year 10 will also cost $100,000.
o Routine maintenance costs are assumed to be the same for all alternates and are ignored in the analysis, except when comparing different structure types such as a buried structure to a traditional bridge. These costs include such activities as minor wearing surface and concrete repairs, yearly cleaning of bearings and drains, and repair of damaged railings.
August 2003 2-12 CHAPTER 2 – PRELIMINARY DESIGN
o User costs are assumed to be the same for all alternates and are ignored in the analysis, unless one alternate has a significant impact on the public over another alternate. User costs can be requested from Planning, if they are used in the analysis.
o Suggested rehabilitation intervals over the life of the bridge are shown in Table 2-2. These may be used as a guide in developing the future rehabilitation over the life of an existing or proposed bridge.
o The Designer should not rely solely on LCCA. The results from LCCA always show deferring costs as the most cost effective solution. However, it is important to consider the additional costs to maintain an old bridge, the impact to the traveling public as a result of additional maintenance work, risks associated with a deteriorating structure, and availability of funding when replacement becomes absolutely necessary. The functionality of the bridge is also important. Replacing a bridge to modern standards may provide an increased bridge width, new sidewalks, or an improved alignment.
Table 2-2 Life Cycle Intervals Useful Life of Component Capital Investment (years) Wearing Surface 15 Replace/Rehab Deck Rehabilitation (includes 30 wearing surface) Deck Replacement 50 Bridge Replacement 75 Painting Refer to Section 7.2.3 Coatings Depends on materials used and Sliplining site conditions Invert Lining 25+ Steel Pipe 50 Plastic Pipe 100 Aluminum Pipe 75 Concrete Pipe/Box 75-100
Notes:
1. Condition of the membrane will determine whether a wearing surface replacement will last 15 years.
2. Extreme traffic or environmental conditions will decrease the useful life of traditional bridges.
August 2003 2-13 CHAPTER 2 – PRELIMINARY DESIGN
3. The substructure can at times outlast the superstructure. The useful life of the substructure should be considered before selecting a rehabilitation alternative.
4. The U.S Army Corps of Engineers document (1997) gives a design life of 50 years for aluminum and plastic pipes. There is evidence that these materials will last much longer.
5. The life of the concrete invert lining is dependent on the longevity of the top plates.
6. The useful life of pipes can vary significantly. Considerations include the cover over the pipe, soil pH and resistively, presence of salts or other corrosive compounds, plate thickness, and flow velocity.
2.2.7 Cost Comparison for Number of Beams
The following discussion is a guide to compare the cost of reducing the number of beams on steel bridges with full cast in place decks only. Future updates to this procedure will include the use of precast deck panels and the use of precast, prestressed beams. Other issues besides cost must be considered as well when determining the optimal number of beams, such as maintenance of traffic during construction and future maintenance needs (refer to Section 7.3 Economy and Section 2.9.6 Maintainability).
For steel beam bridges with relatively wide decks, the Structural Designer may need to investigate the optimum number of beams to use. Fewer beams will result in less total steel required, but will require more deck concrete, and will have slightly higher fabrication costs per pound of steel. A discussion of the cost comparison method is found here.
Regardless of the number or size of the beams, the raw price of steel supplied from the mill can be considered a constant. For this discussion, we assume a cost of $0.50/lb. The cost of fabricating, delivering, erecting, and finishing each beam is relatively independent of the weight of the beam, though will be slightly higher for heavier beams due to issues such as additional welding lengths for deeper webs, larger beam surface area that will require more painting, and thicker plates that will require more effort to drill holes. Therefore, one can assume that this cost for the heavier beam will be approximately 10% higher. If significantly more stiffeners will be required for the heavier beam, this number might be even higher. The ratio of costs will then be the number of beams with narrower beam spacing to the adjusted ratio of the number of beams with wide beam spacing.
Wider beam spacing will also require thicker slabs. When slab thicknesses increase appreciably, the support form costs will increase because of the extra
August 2003 2-14 CHAPTER 2 – PRELIMINARY DESIGN
strength required to carry the extra thickness. However, the added support forms cost will be offset by a decrease in labor cost with fewer beams on which blocking must be formed, and also fewer bays in which support forms must be suspended. Therefore, the cost of forming and finishing is assumed to be equal regardless of beam spacing. The price of concrete delivered and placed can be assumed to be equal to about 35% of the unit price of deck concrete. Generally no cost adjustment is made for reinforcing steel since thicker slabs will have little change in reinforcing steel quantity
The following example illustrates this method of cost comparison.
Example 2-1 Cost Comparison of Number of Steel Beams
Assume a price comparison of four beams to five beams, with a bid price of $1.00/lb for five welded beams, and assuming equal stiffeners on all beams. Weight of steel for 5 beams is 30,000 lb.
ratio of beams = 4/5 = 0.80 ratio of diaphragms = 3/4 = 0.75 assume a cost ratio on fabricating, delivery, and erecting of 0.79, a number chosen between 0.80 and 0.75, but weighted more toward the beam ratio than the diaphragm ratio
5 beams: mill $0.50/lb x 30,000 = $15,000 fab/del/erect $0.50/lb x 30,000 = $15,000 $30,000
4 beams: mill $0.50/lb x 30,000 = $15,000 fab/del/erect $0.50/lb x 0.79 x 1.1 x 30,000 = $13,000 $28,000
Assume a bid price of $450/ yd3 of deck concrete. Assume a five beam bridge will require an 8 inch slab and a four beam bridge will require a 10 inch slab, with quantities of concrete being 150 yd3 and 200 yd3 respectively. The slab costs would be:
8 inch deck: forming & finishing $290 x 150 yd3 = $43,500 delivery & placing $160 x 150 yd3 = $24,000 $67,500
10 inch deck: forming & finishing $290 x 200 yd3 = $58,000 delivery & placing $160 x 200 yd3 = $32,000 $90,000
Summary: 5 beams: $30,000 + $67,500 = $97,500 4 beams: $28,000 + 90,000 = $118,000
August 2003 2-15 CHAPTER 2 – PRELIMINARY DESIGN
2.3 Hydrology, Hydraulics, and Scour
2.3.1 General
Most of Maine’s bridges are located over water. Bridge drainage structures will range from large culvert-type structures to multi-million dollar bridges. Although some hydrologic, hydraulic, and scour analysis is necessary for all bridge drainage structures, the extent of such studies should be commensurate with the complexity of the situation, and with the importance of the structure and of the surrounding property.
Minor spans, bridges, and extraordinary bridges are the responsibility of the Bridge Program.
2.3.2 Minor Span/Strut Determination
Designers must determine on a project-by-project basis if a drainage structure is a strut or minor span. A strut is a structure with a span equal to or greater than 5 feet and less than 10 feet. If a structure has a span equal to or greater than 10 feet, or if multiple structures have a combined opening of at least 80 square feet in area, the structure meets the minimum requirements for a minor span. For a minor span or a bridge, the drainage area is typically 2 square miles or larger with a Q50 flow of 500 cfs or larger. The following examples indicate the minimum flow for a pipe, a pipe arch, and a concrete box that meet the definition of a minor span:
o 10’-3” span by 6’-9” rise steel structural plate pipe arch (18” corner radius) that is 72’ long at 0.5% slope with the end mitered to match the slope (inlet control). HW/D is 0.9 or 90% with approximately 325 cfs.
o 10’ diameter steel pipe that is 72’ long at 0.5% slope with the end mitered to match the slope (inlet control). HW/D is 0.9 or 90% with approximately 525 cfs.
o 10’ span by 10’ rise concrete box culvert that is 72’ long at 0.5% slope with square edge headwall and 0° wingwalls (inlet control). HW/D is 0.9 or 90% with approximately 700 cfs.
Table 2-3 can be used for guidance to determine if a structure is a strut or a minor span based upon an approximate flow.
August 2003 2-16 CHAPTER 2 – PRELIMINARY DESIGN
Table 2-3 Design Flow versus Drainage Area and Wetland Percent Drainage Area (square miles) Wetland % Q50 (cfs) 2 1 549 2 5 409 2 10 287 2 14 211 3 1 753 3 5 563 3 10 388 3 15 269 3 18 215
Note: Flows are based on the U.S.G.S. full regression equation. These values are provided for general guidance and should not be used for hydraulic design purposes.
2.3.3 Level of Analysis
2.3.3.1 Level 1 (Qualitative Analysis)
A Level 1 qualitative analysis involves no numerical analysis. It is used for a project when a pipe or pipe arch is being replaced by another pipe in the same location and when the project meets the following criteria: